1. Field of the Invention
The invention relates to a vehicle braking system for electronically controlling a braking force applied to the vehicle in accordance with a braking amount.
2. Description of Related Art
An electronically controlled braking system has been well known as a braking device for a vehicle that electronically controls the braking force of the braking device, that is, the hydraulic pressure supplied to the wheel cylinder that drives the braking device, in accordance with the braking amount that has been input through the brake pedal. Such braking device has been disclosed in Japanese Patent Application Publication No. JP-A-2004-243983 as described below.
In the vehicle braking system disclosed in Japanese Patent Application Publication No. JP-A-2004-243983, when the driver operates the brake pedal, the master cylinder generates the hydraulic pressure in accordance with the operation amount, and the work fluid partially flows into a stroke simulator such that the operation amount to the brake pedal is adjusted in accordance with the depression force applied to the brake pedal. In the vehicle braking system, the target deceleration of the vehicle is set in accordance with the pedal stroke detected by the brake ECU for determining the distribution of the braking force to the respective wheels such that a predetermined fluid pressure is applied to the respective wheel cylinders.
The vehicle braking control system as described above is provided with a stroke simulator which adjusts the operation amount to the brake pedal by allowing the partial flow of the work fluid to the master cylinder that generates the hydraulic pressure in accordance with the operation amount to the brake pedal. The vehicle braking control system is further provided with a pressurizing mechanism which pressurizes the work fluid supplied to four wheel cylinders via the master cut valve so as to be supplied to the master cylinder in the respective systems. The resultant hydraulic system becomes complicated, thus increasing the manufacturing costs.
In the electronically controlled braking system, the opening degree of the electromagnetic valve is regulated in accordance with the operation amount of the brake pedal so as to apply predetermined fluid pressures to wheel cylinders of the respective wheels. In the case where the battery as the power source installed in the vehicle goes down, the opening degree of the electromagnetic valve cannot be regulated, failing to apply the fluid pressure in accordance with the operation amount to the brake pedal to the wheel cylinders of the respective wheels. In this case, the electronically controlled braking system may be operated using the auxiliary battery provided in the vehicle. This may also cause such problems as the cost increase and deteriorated fuel efficiency owing to the increase in the size and weight of the battery.